Sept. 11 "was an event that forever changed the life of this city," Obama wrote. "And it was a tragedy that will be forever seared in the consciousness of our nation.

"Every year on this day, we are all New Yorkers."

Obama, who actually lived in New York in the early 1980s while studying at Columbia University, observed a moment of silence on the South Lawn of the White House at 8:46 a.m., the moment the first terrorist-hijacked plane crashed into the World Trade Center. In his popignant letter, he recalled the 102 minutes that shocked and horrified the nation eight years ago.

"We will never forget the images of planes vanishing into buildings; of billowing smoke rolling down the streets of Manhattan; of photos hung by the families of the missing.

"We will never forget the rage and aching sadness we felt.

"And we will never forget the feeling that we had lost something else: a sense of safety as we went about our daily lives."

Obama said the anniversary is a reminder of the danger still posed by violent extremists who seek to kill Americans, and said he views keeping the nation safe as his greatest responsibility.

"No one can guarantee that there will never be another attack; but what I can guarantee - what I can promise - is that we will do everything within our power to reduce the likelihood of an attack, and that I will not hesitate to do what it takes to defend America," the president wrote.

He said his plan for ensuring the nation's security has several fronts, including pursuing al Qaeda and Taliban fighters in Pakistan and Afghanistan, investing in the military and intelligence communities, combatting nuclear proliferation and renewing diplomatic efforts.